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EVALUATION OF A HYBRID DRYER FOR THE
PRODUCTION OF APPLE CHIPS
M ADONIS AND MTE KAHN
ABSTRACT
The paper highlights the design, development and evaluation of a hybrid
convective-far-infrared (FIR) dehydrator for the purpose of drying fruit. The
methodology followed is based on an experimental and practical approach.
The application of the dryer for this study is aimed at the production of apple
chips as well as other types of fruit chips and the more common dried fruit. The
fruit dryer proposed uses both infrared energy and convective heating to
effectively and efficiently dehydrate apple slices to produce dried apple chips.
The apple chips are produced in very short time frames and accomplished by
using relatively low heater power. These types of apple chip snacks also
contribute to the more healthy variety of snacks available on the market.
Keywords: Hybrid dryer, infrared-convective dryer, fruit drying, apple chips
1. INTRODUCTION
The global trend in recent years is increasingly towards a healthier lifestyle.
One of the more important aspects of this way of life is nutrition and diet. The
main emphasis is generally on eating foods with a high nutritional value that
has undergone minimal processing (McCallough et al., 2003). Studies on the
various effects that diet has on the physiological well-being of individuals
abound in the literature (Kloranidou et al., 2006; Conway et al., 2002: Popkin
et al., 2001). Many food manufacturers in the snack market have begun
investing in healthier alternative offerings (Thompson, 2006).
Commercially available snack products most often undergo a process of
dehydration to make them suitable for storage and consumption. The
dehydration of foodstuff is one of the oldest methods of food preservation and
by the removal of the majority of water contained in the food, there is an
increase in its shelf-life as well as a reduction of some undesirable reactions
(Ibarz & Barbosa-Canovas, 2000). This is because the micro-organisms
which cause food spoilage and decay are unable to grow and multiply in the
absence of sufficient water and many of the enzymes which promote
undesired changes in the chemical composition of the food cannot function
without water. Water activity is a measure of free moisture in a food and is also
a measure of the availability of water to participate in undesirable chemical
reactions. For the production of apple chips it is necessary to effect the
removal of the enclosed water to a level below that of its water activity.
One of the most common methods for the drying of agricultural products in
developing countries is through the use of solar energy. Solar or sun drying
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is viable on a large scale in tropical areas but has limitations; such as being
weather dependant, contamination of the product by insects and dust and
is also not an easily controllable process. In the developed world however,
drying by means of electrical methods is more common, such as hot-air or
forced convective drying (Vega et al., 2007; Kingsley et al.,2007; Baini &
Langrish, 2007), microwave radiation (Varith et al., 2007) and infrared
radiation (Sharma et al., 2005).
Nomenclature
M
MO
MR
m.c.( w.b.)
m.c.( d .b.)
L
Ad
?
F
t
Much research has been done in various aspects of fruit drying by means of
either infrared radiation, convective or a combination of convection and
infrared drying. The most common method of drying agricultural products is by
means of convective drying. Hot air is passed over the product at varying
temperatures and air velocities. The modeling of the drying of many different
types of fruit and vegetables by means of infrared heating is well documented
in the literature. When infrared energy is used to heat or dry a moist
substance, the radiation impinges upon the exposed substance, penetrates it
and the energy of radiation converts into heat (Hebbar & Rostagi, 2001). In the
thin-layer drying of agricultural substances the dehydration is achieved
through the drying of the substance in a single monotonic layer in a dryer.
Infrared drying has been investigated as a potential method for obtaining high
quality dried foodstuffs, including fruits, vegetables and grains (Abe & Afzal,
1997; Afzal & Abe, 1999; Zhu et al., 2002). The drying of apple slices by
infrared heating was evaluated and models developed which best describe
the physical process (Togrul, 2005). The theoretical drying models reviewed
include the Newton, Page, Modified Page, Wang and Singh, Henderson and
Pabis and the Midilli drying equations amongst others. These theoretical
drying models predict the drying kinetics of the material undergoing drying and
can be of use to industry professionals designing drying equipment for
foodstuff. Infrared drying of apples was compared with convective drying
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Journal for New Generation Sciences: Volume 7 Number 2
alone and the results conclude that with the inclusion of infrared energy, the
drying time is shortened by half (Nowak & Lewicki, 2004).
In the drying of onion slices with an infrared radiation dryer the drying rate
increased with an increase in infrared power at a given air temperature and air
velocity and consequently reduced the drying time (Sharma et al., 2005). The
drying rate was doubled when subjecting carrot to infrared drying, as the
drying temperature was increased from 50˚C to 80˚C (Togrul, 2006). With the
application of far-infrared radiation (FIR) drying to the paddy drying process it
was shown that FIR radiation was very effective in moisture reduction of wet
paddy and the temperature inside a paddy grain also increased with FIR
intensity level (Meeso et al., 2007).
The combined mode of heat transfer incorporating infrared radiation and
forced convection drying has been reported. In the drying of apple pomace
with the incorporation of forced convection as a method of pre-drying, the
drying time was reduced giving a time saving of 20% for a process without
convective pre-drying (Sun et al., 2007). For the drying of carrot and potato the
performance evaluation studies revealed that the combination drying reduced
the drying time by 48%, consumed less energy in the process requiring only
63% and the energy utilisation efficiency of the dryer was estimated to be 38%
(Umesh Hebbar et al., 2004). In the study on the numerical modeling of the
hygrothermal behaviour of a rectangular-shaped porous material during
combined drying, it was shown that when compared with convective drying
alone there is a rapid superficial drying with some materials drying in less than
30 minutes (Salagnac et al., 2004). The drying time also decreased in the thin
layer drying of onion slices under infrared and convective drying (Jain &
Pathare, 2004).
The purpose of this paper is to highlight the development of a hybrid
convective-far-infrared batch-type dryer for fruit. An overview of the theory of
fruit drying is explored, as well as a brief overview of the theory of drying, and
the drying kinetics curves including the results of drying tests performed in
order to evaluate the dryer are presented. The dryer developed, although only
tested using apple slices, is designed to be adaptable for a range of
agricultural products requiring drying to various final moisture contents.
2. DRYING THEORY OVERVIEW
2.1
Types of water found in food
The absorption of water by an organic, chemically inert material is a complex
process which is not entirely understood. This complexity becomes much
greater when biological materials are involved. This complexity is due
principally to the fact that water may be present in several different forms
(Earle & Earle, 1983).
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2.1.1 Water of hydration
This moisture is chemically bound to the constituents of the material
and in most cases would not be considered in moisture content
determinations. It is considered to be an integral part of the material.
2.1.2 Bound water
Water which is in some way bound to the food so that it exerts a vapour
pressure less than that of pure water. Can be bound by dissolved
substances (osmotic pressure, suction), chemical bonding (this is not
very important in food systems), by the action of capillaries and
hydrogen bridges, van der Waals forces and ionic and polar bonds. It
can often be thought of as the first layer of water molecules attached to a
surface.
2.1.3 Free water
Free water is water which is bound by such minute forces that its
vapourpressure is equal to the vapour pressure of pure water. It can be found
as free water in cavities and wide capillaries. This can often be thought of as
the second and subsequent layers of moisture attached to a surface. The heat
of adsorption of this moisture is equal to the normal heat of vaporisation of
water at the same temperature.
2.1.4 Absorbed moisture
This is moisture that has passed through cell walls and entered the cytoplasm
of the cell. It is this form of water that is believed to account for the hysteresis
between the sorption and desorption equilibrium moisture content isotherms.
2.2
MECHANISM OF DRYING
During drying it is necessary to remove free moisture from the surface and
also moisture from the interior of the material. During drying, heat is
transferred to the surface, and the latent heat of vaporisation causes water to
evaporate. Water vapour diffuses through a boundary film of air and is carried
away by the moving air. This creates a region of lower water vapour pressure
at the surface of the food, and a water vapour pressure gradient is established
from the moist interior of the food to the dry air. This gradient provides the
driving force for the removal of water from the food. Water moves to the
surface by the following mechanisms:
•
•
•
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liquid movement by capillary forces;
diffusion of liquids, caused by differences in the concentration of solutes
indifferent regions of the foods;
diffusion of liquids which are adsorbed in layers at the surfaces of solid
Journal for New Generation Sciences: Volume 7 Number 2
•
components of the food; and
water vapour diffusion in air spaces within the food caused by vapour
pressure gradients.
Dry ng rate
A
B
C
time
Figure 1: Typical drying curve
The drying rates for different foods will have a wide range of variation, but
atypical curve is shown in Figure 1. When the food is placed inside the drier,
there is a short settling down period as the surface heats up to the wet bulb
temperature (region A). Drying then commences along AB, and provided that
water moves from the interior of the food material at the same rate as it
evaporates from the surface, the surface remains wet. This period is known as
the constant rate period, as can be seen from the drying curve: this is because
drying occurs over a constant rate over the period A to B. The constant rate
period continues until the critical moisture content is reached. At this point
drying enters what is known as the falling rate period (region B to C).
Food systems however, present many challenges, as in practice different
areas of the surface of the food will dry out at different rates and overall the rate
of drying declines gradually during the 'constant rate period'. In some cases it
is accepted that the constant rate period does not exist. Thus the critical point
is not fixed for a given food and depends on the amount of food in the drier and
the rate of drying. The three characteristics of air that are necessary for
successful drying in the constant rate period are: a moderately high dry bulb
temperature; a low relative humidity; and a high air velocity.
The boundary film of air surrounding the food acts as a barrier to the transfer of
both heat and water vapour during drying. The thickness of the film is
determined primarily by air velocity. If this is too low, water vapour leaves the
surface of the food and increases the humidity of the surrounding air, to cause
a reduction in the water vapour pressure gradient and the rate of drying.
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3. MATERIAL AND METHODS
3.1
The experimental dryer
The dryer designed and developed for this study is based on a hybrid design
that incorporates the use of infrared radiation and forced convection heating.
The design is based on a batch-type dryer with a removable tray. The types of
infrared heaters used are the far-infrared ceramic type and are arranged in a
heater bank inside the dryer cavity directly above the tray. The distance from
the FIR heater bank to the tray is fixed at 24cm. Figure 2 illustrates the
constructed experimental hybrid FIR-convective dryer developed. The
convective heater is a separate metal-sheath type in conjunction with a fan,
positioned at the rear of the unit. The installed electrical power capacity of the
infrared heaters is 16 x 400 watt heaters and 1 x 2500 watt convective heater.
The total installed electrical power capacity of the hybrid dryer is
approximately 9 kilowatts. The dryer cavity is not heat insulated and is not
enclosed by an access door.
Figure 2: Experimental hybrid IR-convective dryer
3.2
The experiments
Apples (Golden Delicious) were purchased from a local fresh produce market
and allowed to acclimatise in a refrigerator to 4°C for approximately two hours.
Apples of similar size and mass were sliced into widths of between 4 mm and 6
mm and the widths were measured and verified using vernier callipers. The
slices of similar size were arranged on the tray, completely filling it, and placed
inside the dryer. The experiments were conducted in triplicate and in order to
obtain the drying kinetics data, periodic sampling or weighing of a sample in
each experiment was adopted. The complete batch of apple slices per
experiment was approximately 280 grams. Individual samples were
intermittently removed from the dryer at fixed time intervals and weighed on an
electronic mass balance with a resolution of 0.01g. In order to prevent
enzymatic browning and prolong shelf life it is common practice for the
commercialisation of dried fruit to treat it with either a solution of lemon juice or
citric acid; however the apple slices used in these experiments were
untreated.
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The dryer was preheated to the specifications as indicated in Table 1 below.
Table 1: Dryer specifications
Experiment
Air velocity,
average
(m/s)
Air
FIR Heater
FIR Heater
temperature, input voltage percentage
average (°C) (V)
total power (%)
1
2
3
0.4
0.4
0.4
60
60
60
80
100
120
12
19
27
The air velocity and air temperature were kept constant throughout and only
the infrared heater parameters were modified. The initial relative humidity was
measured at between 40 and 45% throughout each of the experiments. The
final relative humidity was not determined.
The initial moisture content of the apples was not determined intrinsically.
Instead the value was estimated based on available data. Percentage
moisture content (wet basis) is derived from the following relation:
(1)
According to Earle and Earle (1983), the moisture content of apple was
measured as 84% w.b. and in another study, Sacilik and Elicin (2006)
determined the value to be approximately 82% w.b. For the purposes of the
present study the moisture content of the apple slices used was estimated to
be approximately 80% w.b.
However, the moisture content based on the dry weight basis is used in the
calculations and the drying kinetics curves plotted from the experimental data
obtained. Percentage moisture content (dry basis) is given by the formula:
(2)
A further quantity of interest in drying is the moisture ratio (MR), a
dimensionless quantity, described as a ratio that compares the mass or
volume of air to the mass or volume of moisture contained in that air and is
expressed as the following:
(3)
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In infrared drying the sample may be dried as much as dried matter content
(Togrul, 2005). The material dried in the hybrid dryer was subjected to infrared
radiation intensities as indicated in Table 2 below.
Table 2: Hybrid dryer radiation intensities
Infrared heater voltage (V)
Radiation intensity (kW/m²)
80
100
120
2.47
3.87
5.57
4. RESULTS AND DISCUSSION
The results of the drying experiments performed with the hybrid dryer indicate
that the drying of the apple slices occurred exclusively in the falling-rate
period. There was no part of the drying happening in the constant-rate period.
An indication of the mass variation of each of the samples over time in the
drying experiments performed is shown in Figure 3.
Figure 3: Mass of samples vs. time
Each of the samples tended towards a constant mass the end of the
experiments. This steady mass is an indication that the final moisture content
was reached and that most of the moisture present in the samples was
removed. Further drying is possible but the samples are in jeopardy of being
burnt and damaged. The variation of the moisture content (d.w.b) of each of
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Journal for New Generation Sciences: Volume 7 Number 2
the samples is given in Figure 4. Analysis of the curves indicates that the
higher the infrared heater power (experiment 3) the quicker the drying occurs
even with a sample having of higher initial moisture content. The gradient of
the drying curve is initially very steep. Consequently drying at lower infrared
heater temperature shows a longer drying time and has a gradual decreasing
slope. Of interest here is whether such steep temperature gradients have an
adverse effect on the agricultural product being dried, to such an extent that
this results in undesirable effects on the quality of the product, taste, texture,
colour and nutritional content. These studies are the subject of further
research that incorporate the hybrid dryer.
Figure 4 provides evidence of the drying of the apple slices occurring only in
the falling-rate period. The drying time takes place in a range from
approximately 26 minutes (27% infrared power) to 40 minutes (12% infrared
power). The drying experiments were performed until a moisture content
(w.b.) of approximately 15% was reached. Although the drying was allowed to
continue until a final moisture content of less than 10% was reached, many of
the samples had turned brown and showed signs of damage and spoilage at
this point. Therefore, although apple slices can be dried to lower and lower
final moisture contents, the agricultural product is not always able to withstand
the strong temperature gradients and vapour pressures exerted on it and
consequently the samples can be completely denatured and ruined as a
commercially viable product.
Figure 4: Moisture content vs. time
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An important quantity in the evaluation of the effectiveness of a dryer design is
the variation of moisture ratio, which is derived from the experimental moisture
content data, versus drying time. The deviation of moisture ratio with drying
time for the samples is indicated in Figure 5. The variation of the drying rate
with drying time is shown in Figure 6. These curves reveal the faster drying
achieved with the higher infrared heater power. However, the risk at a higher
infrared power is the possible damage to the fruit that could result.
Careful control mechanisms need to be implemented in order to prevent this
from occurring. This could be in the form of a control action, which varies the
infrared heater power in comparison to measurements of either moisture
contents, colour recognition of the fruit surface or a time factor.
An evaluation of the combined infrared and convective hybrid dryer efficiency
is derived and expressed as the heat utilisation efficiency (η) and given by the
formula:
(4)
Nominal values derived from the drying process are indicated in Table 3. The
high values indicated in Table 3 are possible because relatively small batches
of apple slice samples were loaded into the dryer. Previous values for heat
utilisation efficiencies were found to be of the order of 36% and 39% for the
combined infrared and hot air drying of vegetables (Umesh Hebbar et al.,
2004).
The dryer is capable of being modified to incorporate air recirculation to
possibly further increase the drying efficiency. Figure 7 provides a thermal
image of the drying temperatures (deg C) present on an apple slice and near
the dryer interior during a drying cycle. An indication of the quality of the dried
apple slices achieved with the use of the hybrid dryer is shown in Figure 8.
Figure 9 further indicates the result of drying apple samples to lower and lower
moisture contents with the consequence of browning and spoiling the final
dried product.
Table 3: Values of heat utilisation efficiency per drying experiment
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Experiment
Heat utilisation efficiency (%)
1
2
3
62
55
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Journal for New Generation Sciences: Volume 7 Number 2
Figure 5: Moisture ratio vs. time
Figure 6: Drying rate vs. drying time
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Figure 7: Thermal image of apple slice being dried
Figure 8. Sample of dried apple slice
Figure 9: The result of drying samples to lower moisture contents
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5. CONCLUSION
The hybrid infrared convective dryer presented is able to dehydrate apple
slice samples from an initial moisture content of 80% to final moisture content
of 15% and lower. Small batches of untreated apple slices were dried in
relatively short drying times and at moderately high dryer efficiencies. The
results obtained from the experiments using the dryer show some promise
and further refinement of the techniques involved should produce a fully
commercially viable artifact. Modifications of the device for a range of
agricultural products would extend the usefulness of this apparatus in the
dehydration of fruits and vegetables and such a device could be of value to
producers in the snack market.
6. REFERENCES
Abe, T. & Afzal, T. 1997. Thin-layer infrared radiation drying of rough rice.
Journal of Agricultural Engineering Research 67:289-297.
Abe, T. & Abe, T.M. 1999. Some fundamental attributes of far infrared radiation
drying of potato. Drying Technology 17:137-155.
Baini, R. & Langrish, T. 2007. Choosing an appropriate drying model for
intermittent and continuous drying of banana. Journal of Food Engineering
79:330-343.
Conway, T., Sallis, J., Pelletier, R., Powers, H., Marshall, S., Zive, M. & Elder,
J. 2002. What do middle school children bring in their bag lunches? Preventive
Medicine 34:422-427.
Earle, R. & Earle, M. 2004. Unit operations in food processing - the web
edition. Available at: http://www.nzifst.org.nz/unitoperations/index.htm
(accessed 20 May 2008).
Hebbar, H. & Rostagi, N. 2001. Mass transfer during infrared drying of cashew
kernel. Journal of Food Engineering 47:1-5.
Ibarz, A. & Barbosa-Canovas, G.V. 2000. Unit operations in food engineering.
New York, CRC Press.
Jain, D. & Pathare, P. 2004. Selection and evaluation of thin-layer drying
models for infrared radiative and convective drying of onion slices.
Biosystems Engineering 89:289-296.
Kingsley, A., Singh, R., Goyal, R. & Singh, D. 2007. Thin-layer drying
behaviour of organically produced tomato. American Journal of Food
Technology 2:71-78.
20
Kloranidou, V., Papadopoulou, S., Fahantidou, A. & Hassipidou, M. 2006.
Physical activity effect on snacks choice of children. Nutrition and Food
Science 36:400-406.
McCallough, F., Jones, S. & Vignali, D. 2003. The pot snack market - are
today's consumers demanding health as well as convenience? British Food
Journal 105:395-404.
Meeso, N., Nathakaranakule, A., Madhiyanon, T. & Soponronnarit, S. 2007.
Modeling of far-infrared irradiation in paddy drying process. Journal of Food
Engineering 78:1248-1258.
Nowak, D. & Lewicki, P. 2004. Infrared drying of apple slices. Innovative Food
Science and Emerging Technologies 5:353-360.
Popkin, B., Siega-Riz, A., Haines, P. & Jahns, L. 2001. Where's the fat? Trends
in U.S. diets 1965-1996. Preventive Medicine 32:245-254.
Sacilik, K. & Elicin, A. 2006. The thin layer drying characteristics of organic
apple slices. Journal of Food Engineering 73:281-289.
Salagnac, P., Glouannec, P. & Lecharpentier, D. 2004. Numerical modeling of
heat and mass transfer in porous medium during combined hot air, infrared
and microwaves drying. International Journal of Heat and Mass Transfer
47:4479-4489.
Sharma, G., Verma, R. & Pathare, P. 2005. Mathematical modeling of infrared
radiation thin layer drying of onion slices. Journal of Food Engineering 71:282286.
Sun, J., Hu, X., Zhao, G., Wu, J., Wang, Z., Chen, F. & Liao, X. 2007.
Characteristics of thin-layer infrared drying of apple pomace with and without
hot air pre-drying. Food Science and Technology International 13:91-97.
Thompson , S. 2006. Frito-Lay wants to help you eat your five a day.
Advertising Age 77 12.
Togrul, H. 2005. Simple modeling of infrared drying of fresh apple slices.
Journal of Food Engineering 71:311-323.
Togrul, H. 2006. Suitable drying model for infrared drying of carrot. Journal of
Food Engineering 77:610-619.
Umesh Hebbar, H., Vishwanathan, K. & Ramesh, M. 2004. Development of
combined infrared and hot air dryer for vegetables. Journal of Food
Engineering 65:557-563.
21
Journal for New Generation Sciences: Volume 7 Number 2
Varith, J., Dijkanarukkul, P., Achayaviriya, A. & Achariyaviriya, S. 2007.
Combined microwave-hot air drying of peeled longan. Journal of Food
Engineering 81:459-468.
Vega, A., Fito, P., Andres, A. & Lemus, R. 2007. Mathematical modeling of hotair drying kinetics of red bell pepper (var. Lamuyo). Journal of Food
Engineering 79:1460-1466.
Zhu, K., Zou, J., Chu, Z. & Li, X. 2002. Heat and mass transfer of seed drying in
a two pass infrared radiation vibrated bed. Heat Transfer-Asian Research
31:141-147.
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